The present invention relates to a magnetic brush development apparatus for developing a latent electrostatic image for use with an electrophotographic copying apparatus and an electrostatic recording apparatus.
In the conventional development apparatus of a magnetic brush development type, a latent electrostatic image is developed by one development roller which is capable of scooping up a developer. Recently, however, there has been a great need and demand for a high speed copying and maintenance-free machine on the market. By a "maintenance-free" copying machine here is meant a machine which can be used without frequent attention to maintenance operations on the machine, such as exchanging the developer. In order to meet such a demand on the market, it would appear to be necessary to increase the quantity of the developer that can be held in the development apparatus. Of course, there could be some other preferable way to meet the demand on the market, such as by improving the developer. However, it has not been accomplished yet. In the method of increasing the quantity of the developer that can be held in the development apparatus, the aim is to reduce the number of circulation cycles of the developer by increasing the quantity of the developer in the development apparatus, so that a fatigue of the developer, particularly a fatigue of carriers, is obviated as much as possible. The fatigue and deterioration of the carrier is promoted by the contact of the carriers with other materials and the carriers themselves. The developer which has been used in development is rested for a while. Therefore, in the development apparatus of a high speed copying machine, the developer is circulated at a high speed, but the quantity of the developer held in the development apparatus is so large that a considerable change of the concentration of toner in the developer does not occur and the fatigue and deterioration of the developer can be reduced and, accordingly, the period between the exchange of the developer can be considerably lengthened. However, as the quantity of the developer held in the development apparatus is increased, the development apparatus has to become oversized. In this case, when one development roller is employed as in the conventional development apparatus, a long development roller has to be employed. As a result, the length of the development roller causes a difficult problem in the overall layout of a copying macine employing such a development roller. Furthermore, in this case, the developer has to be carried from a distant portion with stirring by a developer transporting apparatus. During the transportation, the developer receives a mechanical friction and is deteriorated. In particular, in a dry type development apparatus, all of the developer has to be circulated. Otherwise, the developer coagulates and accordingly it cannot be used any longer.
Under such circumstances, the development apparatus as illustrated in FIGS. 1 and 2 have been proposed.
In FIG. 1, there is shown a multi-magnetic brush development apparatus. Reference numeral 1 indicates a photoconductor drum which is rotated in the direction of the arrow. A development apparatus 2 comprises three development rollers 3, 4 and 5, each of which comprises a non-magnetic sleeve and a magnet and which are rotatable in the direction of the respective arrows in close proximity to the photoconductor drum 1. The development roller 3 scoops up the developer placed in a bottom portion of the development apparatus 2 and supplies the developer to the development roller 4, which developer is transferred from the development roller 4 to the development roller 5 and from all of the rollers to drum 1, so that a latent electrostatic image on the photoconductor drum 1 is developed by the development rollers 3, 4 and 5. The developer that has not been used in the development is scraped from the development roller 5 by a developer scraping plate 6 and the thus scraped developer is dropped into a developer holding portion 7. A predetermined quantity of the developer is then supplied to the development roller 3 through a gap 7a from the developer holding portion 7. In this development apparatus, it is difficult to keep the amount of the developer constant on each development roller since there is no doctor member before the development station. In particular, in the method for transporting the developer from one development roller to the other development roller, the magnetic brush of the developer on each development roller gradually becomes higher and accordingly the bulk density of the toner of the magnetic brush is lowered. Therefore, the gap between each development roller and the photoconductor drum 1 has to be adjusted. Thus, the magnetic transport of the developer is apt to bring about an unstable flow of the developer and it is difficult to make a uniform magnetic brush and to perform a uniform development.
Futhermore, a development apparatus 10 as shown in FIG. 2 has been proposed. In FIG. 2, the development apparatus 10 comprises one development roller 8 and a developer feed roller 9. The developer is magnetically transported from the developer feed roller 9 to the development roller 8. Also in this case, it is difficult to form a uniform magnetic brush on the development roller 8. In order to remove the difficulty, it has been proposed to dispose a doctor member in close proximity to the developer feed roller 9. However, even with such an arrangement it is difficult to make the height of the magnetic brush uniform at the time of development.
As a result, it might be proposed to dispose a doctor member in close proximity to the development roller 8 of the development apparatus 10 in FIG. 2. In such case, any excessive developer removed by the doctor member will gradually build up and there is no room for the removed developer, and a great load is applied to the development roller 8 and to the developer itself, so that the deterioration of the developer is promoted.
Referring to FIG. 5, there is shown a further conventional development apparatus. In the figure, a development apparatus 102 is disposed under the left side of a photoconductor drum 101, and a magnetic brush is formed on a development roller 103 having a magnet therein and a latent electrostatic image on the photoconductor drum 101 is developed by the magnetic brush. In a developer container 104 for holding a developer therein, two doctor members 105 and 106 are disposed so as to face the development roller 103. The doctor member 105 serves to regulate the quantity of the developer supplied to the development roller 103, while the doctor member 106 regulates the amount of the developer suitably for performing development in a gap between the photoconductor drum 101 and the development roller 103. Another role of the doctor member 105 is to prevent the reactive torque on the development roller 103 from increasing by the building up of the developer between the development roller 103 and the developer container 104 when an excessive amount of the developer is removed from the development roller 103 by the doctor member 106.
Referring to FIG. 6, there is shown a still further conventional development apparatus. In the figure, under a development roller 107, there is disposed a developer feed roller 108, which magnetically scoops up the developer placed in a developer container 109 and transports magnetically all of the scooped developer to the development roller 107. In this development apparatus, a doctor member 110 is disposed in close proximity to the development roller 107 before a development station so that the quantity of the developer for development is regulated. However, since the magnetic developer on the development roller 107 is magnetically transported against its weight from the developer feed roller 108, the bulk density of the magnetic brush is lowered at the time of development even if the height of the magnetic brush is regulated by the doctor member 110.
In the case of a developer comprising resincoated magnetic carriers and a toner, the carriers are expanded by an electrostatic repulsion between the carriers while the developer is stirred. As a result, the bulk density of the developer decreases. The inventor of the present invention stirred a developer comprising the coated carriers and 1.5 weight percent of toner and measured the bulk density of the developer. As shown in FIG. 7, the bulk density of the developer became smaller with time by the expansion of the developer and, in about 30 minutes, the expansion of the developer was saturated. The bulk density of the developer is also decreased while the developer is scooped up and transferred between the magnetic rollers and is caused to pass over a plurality of magnetic poles. In the development apparatus as shown in FIG. 2, the expansion of the developer occurs even in the case of a two-component type developer comprising an ordinary toner and iron powders since the developer is scattered within the development apparatus and accordingly the bulk density of the developer decreases.
When the bulk density of the developer decreases, the magnetic brush formed by the developer becomes coarse and non-uniform development and insufficient image density occur.